Submembrane granular fusion to cell membranes appears to be driven by granular osmotic pressure, leading to vesicular swelling and membrane merger at the contact between granular vesicle and cell membrane. We have performed experiments to learn how this swelling is accomplished. Compromising the integrity of the granule membrane to the extent of allowing the passage of small molecules does not affect fusion. Exocytosis proceeds without ions. Polymers prevent exocytosis by preventing the disposal of the granule contents once fusion has occurred. It seems that calcium, in triggering exocytosis, triggers some alteration in the state of the internal granule phase to increase the affinity of this phase for water.